Mechanical properties play an important role in regulating cellular activities and are critical for unlocking the mysteries of life. Atomic force microscopy (AFM) enables researchers to measure mechanical properties of single living cells under physiological conditions. Here, AFM was used to investigate the topography and mechanical properties of red blood cells (RBCs) and three types of aggressive cancer cells (Burkitt's lymphoma Raji, cutaneous lymphoma Hut, and chronic myeloid leukemia K562). The surface topography of the RBCs and the three cancer cells was mapped with a conventional AFM probe, while mechanical properties were investigated with a micro-sphere glued onto a tip-less cantilever. The diameters of RBCs are significantly smaller than those of the cancer cells, and mechanical measurements indicated that Young's modulus of RBCs is smaller than those of the cancer cells. Aggressive cancer cells have a lower Young's modulus than that of indolent cancer cells, which may improve our understanding of metastasis.